The evolution of limb function from theropods to birds

[GUY LEAHY <xrciseguy@q.com>]

Naturwissenschaften. 2008 Dec 24.
The evolutionary continuum of limb function from early theropods to 
birds.Hutchinson JR, Allen V.
Structure and Motion Laboratory, Department of Veterinary Basic Sciences, The 
Royal Veterinary College, University of London, Hatfield, Hertfordshire, AL9 
7TA, UK, jrhutch@rvc.ac.uk.
 
The bipedal stance and gait of theropod dinosaurs evolved gradually along the 
lineage leading to birds and at some point(s), flight evolved. How and when did 
these changes occur? We review the evidence from neontology and palaeontology, 
including pectoral and pelvic limb functional morphology, fossil 
footprints/trackways and biomechanical models and simulations. We emphasise 
that many false dichotomies or categories have been applied to theropod form 
and function, and sometimes, these impede research progress. For example, 
dichotomisation of locomotor function into 'non-avian' and 'avian' modes is 
only a conceptual crutch; the evidence supports a continuous transition. 
Simplification of pelvic limb function into cursorial/non-cursorial 
morphologies or flexed/columnar poses has outlived its utility. For the 
pectoral limbs, even the classic predatory strike vs. flight wing-stroke 
distinction and separation of theropods into non-flying and flying-or 
terrestrial and arboreal-categories
 may be missing important subtleties. Distinguishing locomotor function between 
taxa, even with quantitative approaches, will always be fraught with ambiguity, 
making it difficult to find real differences if that ambiguity is properly 
acknowledged. There must be an 'interpretive asymptote' for reconstructing 
dinosaur limb function that available methods and evidence cannot overcome. We 
may be close to that limit, but how far can it be stretched with improved 
methods and evidence, if at all? The way forward is a combination of techniques 
that emphasises integration of neontological and palaeontological evidence and 
quantitative assessment of limb function cautiously applied with validated 
techniques and sensitivity analysis of unknown variables.
 
Guy Leahy